Heretofore, a thermo-element using a thermally expandable material, such as paraffin, as a temperature sensor, has been employed in various devices. The thermo-element is operable to convert a change in volume of paraffin due to a thermally-caused phase change thereof to a linear movement of a piston in an axial direction thereof. The thermo-element includes a diaphragm type as illustrated in FIG. 1, and other types, such as a sleeve type and a thick sealing member type (not illustrated). The diaphragm type thermo-element is configured such that a thermally expandable material (paraffin) is hermetically sealed by an elastic sealing member, and a piston is pushed out by the elastic sealing member through an incompressible fluid. Each of the sleeve type and the thick sealing member type thermo-elements is configured such that a thermally expandable material (paraffin) is hermetically sealed by an elastic sealing member, and a piston is pushed out directly by the elastic sealing member.
In a diaphragm type thermo-element illustrated in FIG. 1, a cylindrical-shaped guide member 5 is fixed to a bottomed cylindrical-shaped casing 1. A thermally expandable material 2 is filled in the casing 1, and an upper end surface of the thermally expandable material 2 is sealed up by a diaphragm 3 serving as an elastic sealing member. A liquid chamber 4 is provided between a contact surface 5a which is an inner surface of a base end portion of the guide member 5, and an upper surface of the diaphragm 3, and filled with a gum-like fluid composed of a deformable, incompressible fluid. A rubber piston 7, a protection plate 8 and a piston 6 are provided inside a sliding hole 5b of the guide member 5 and above the liquid chamber 4 in this order. An upper portion of the piston 6 protrudes upwardly from the sliding hole 5b. 
Upon an increase in ambient temperature, the thermally expandable material 2 expands, so that the diaphragm 3 is raised upwardly to push up the gum-like fluid enclosed in the liquid chamber 4 above the diaphragm 3. Then, the gum-like fluid is deformed to flow into the sliding hole 5b and push the piston 6 upwardly through the rubber piston 7 and the protection plate 8. Subsequently, upon a decrease in ambient temperature, the thermally expandable material 2 contracts, so that the piston 6 is pushed down by a load (not illustrated) applied thereto. In this way, the piston 6 is relatively moved with respect to the guide member 5 in an up and down direction, in response to temperature changes.
However, the diaphragm type thermo-element has a problem that the incompressible fluid is liable to leak, due to its structural complexity. Even in the sleeve type thermo-element, the same problem occurs when it has a structure employing an incompressible fluid.
In the case of sealing a device which employs a commonly-used fluid, an O-ring or V-ring is used, and grease is applied to a sliding surface in contact with the O-ring or the like to reduce sliding resistance. The device is operated while supplying supplementary grease, because grease leakage inevitably occurs. However, a commonly-used O-ring or the like cannot be used in the thermo-element, because, if grease leakage occurs, a volume of the incompressible fluid is changed, which exerts an influence on an amount of movement of the piston.
For this reason, in the diaphragm type thermo-element, the thermally expandable material is isolated by a diaphragm, and a gum-like fluid free of liquefaction and less likely to leak is used as the incompressible fluid. Further, a rubber piston having excellent sealing performance is used as a sealing member, instead of an O-ring.
The gum-like fluid is intended to transfer expansion of the thermally expandable material to the piston, by using a material less likely to leak, so that it has poor fluidity as compared to liquid. Thus, a shape of the liquid chamber 4 for enclosing the gum-like fluid therein is restricted to a simple shape.
As a countermeasure against leakage of the gum-like fluid, studies have heretofore been made, for example, of view such as a structure of the thermo-element and a material for the gum-like fluid.
JP32225386 relates to an improvement in structure of the diaphragm type thermo-element, and discloses a structure in which a piston is inserted in a gum-like fluid composed of an incompressible fluid, and a gap between a guide member and the piston is hermetically sealed by a packing, thereby allowing for elimination of a rubber piston and a protection plate.
JP U58-016003 (Japanese Utility Model Publication) relates to an improvement of the sleeve type thermo-element, and discloses a structure in which a sealed chamber is defined between a sleeve and a piston, and filled with an incompressible fluid, wherein the sleeve is adapted to push up the piston through the incompressible fluid.
In JP3225386 and JP U58-016003, there still remains a problem of leakage of the gum-like fluid composed of an incompressible fluid, and each of the thermo-elements becomes structurally more complex.
JP U06-043591 (Japanese Utility Model Publication) relates to an improvement in the material of a gum-like fluid composed of an incompressible fluid, and discloses a gum-like fluid formed as a semifluid by finely crushing synthetic gum, mixing the resulting crushed gum pieces with a lubricant such as grease or oil, and kneading them. However, this semifluid is likely to give rise to a phenomenon lading to a locked state, such as blocking caused by the crushed gum pieces.
JP 11-293235 discloses that paraffin as the thermally expandable material is absorbed in carbon black particles and prepared in a powder form to reduce the risk of leakage of the paraffin, and that a piston is directly inserted in the thermally expandable material and adapted to be pushed up directly by the thermally expandable material due to a change in volume of the thermally expandable material. In the JP 11-293235, the paraffin absorbed in the carbon black particles is prepared in a powder form. Thus, an operation of the thermo-element is liable to become unstable, although the paraffin is sealed by a sealing member.
As stated above, in a thermo-element adapted to move a piston through an incompressible fluid according to expansion and contraction of a thermally expandable material, irrespective of whether it is a diaphragm type or the sleeve type, the problem of how to prevent leakage of the incompressible fluid (gum-like fluid) has not yet been sufficiently solved.
Therefore, there is a need for a thermo-element capable of stably operating without occurring of the problem of leakage of an incompressible fluid and with small sliding resistance of a piston.
There is another need for a thermo-element having excellent durability.
There is yet another need for a thermo-element having fewer restrictions on shape.